Automated fiber placement devices form composite parts by applying a plurality of narrow strips of material (e.g., carbon fiber impregnated with resin) referred to as tows. An automated fiber placement machine may apply tows by depositing the tows with a roller or other device. The automated fiber placement device applies or places multiple adjacent tows to form a contiguous array or band, referred to as a course. Multiple courses are applied to form a single layer of tows, referred to as a ply. A composite part includes multiple plies (a set of plies), and plies of the set of plies are often oriented in different directions (e.g., a 0 degree ply, a +45 degree ply, a 90 degree ply, and a −45 degree ply). An orientation of the ply refers to a nominal direction of the tows (and the composite fibers thereof) of the ply.
During tow application, automated fiber placement devices can start or stop individual tows within a course at designated locations. To stop a tow, the automated fiber placement device cuts the tow transversely and dispenses no further tow material. This is commonly referred to as “dropping” the tow. A corresponding blunt tow-end formed during dropping the tow is called a “tow drop.” “Tow drop” may also refer to a blunt tow-end formed when starting deposition of a tow. Automated fiber placement devices can generally start or stop individual tows of a course at different locations to increase or decrease the width of a particular course as the particular course is applied or to make the tows conform to a specified ply outline. Each tow of the course has a fixed width; thus, when forming a curved part or a part having taper, some overlaps (locations where one tow overlies another tow) and gaps (voids in between two tows) may be formed in the plies. The region of a ply where courses come together obliquely and where tow drops are performed in order to prevent a surfeit or paucity of material and where gaps and overlaps are formed as a result of the blunt-ended geometry of tow drops, described below, is called a convergence zone. The gaps and overlaps of each ply create thickness variations (deviations from a nominal thickness) in the composite part. The thickness variations which manifest as depressions (from gaps) and bumps (from overlaps) in the surface of the composite part may be mitigated to achieve a desired tolerance, performance, or aesthetic, by post processing steps such as shimming, filling, grinding, polishing, machining, etc. These post processing steps may add weight to a part or cost and time to its production. Additionally, mitigating the thickness deviations may degrade strength and wear characteristics of the composite part.
Conventional methods of applying tows either result in relatively small magnitude (e.g., about 1 ply thickness) thickness deviations from the nominal thickness being spread over a relatively large region (e.g., multiple tow widths wide) of the part or result in relatively large magnitude (e.g., greater than 1 ply thickness) deviations from the nominal thickness being confined to a relatively small region (e.g., about 1 tow width wide) of the part. With each of the conventional methods, adding plies (forming thicker parts) increases an area of the region containing thickness variations or a magnitude of the thickness variations contained therein.